Eg, seaweed (4)
I believe the answer is:
alga
'eg seaweed' is the definition.
(seaweed is a kind of alga)
This is the entire clue.
(Other definitions for alga that I've seen before include "Seaweed family group" , "Plants such as seaweed" , "Eg, seaweed" , "Primitive non-flowering, mainly aquatic, plant" , "Autotrophic organism" .)